Many modern industrial buildings are constructed by first constructing a strong steel framework and then cladding the same with thin profiled metal sheets so as to form the walls and roof.
It is also common practice to add insulation material to the walls and roof of the building so as to conserve energy and to help to maintain a comfortable environment within the building. This added insulation is usually sandwiched between the roof or walls and the framework as the roof and walls are connected to the underlying framework by using self-drilling, self-tapping screw-threaded fasteners. A typical building has many thousands of such fasteners.
Inside a typical air-conditioned building there is usually a small positive pressure which forces warm moist air from the interior of the building along the screw threads and into the wall and roof voids, as well as into the insulation material. The temperature outside of the roof void and the temperature gradient within the roof void vary according to location, the time of year and the time of day but, frequently, the void will be cooler than the inside of the building.
The warm air from the building therefore cools within the roof and wall voids and consequently moisture condenses. This condensation typically reacts with chemicals present within the insulation material so as to form an acid which subsequently attacks the fastenings. This chemical or corrosive attack can lead to very severe corrosive deterioration of the fastenings and the structural integrity of the associated structural framework and network. Alternatively, condensation can accumulate and saturate the insulation so as to reduce its thermal efficiency. In the most severe cases, the condensation flows back into the building and gives the impression of being leakage.
Corrosion is a frequent cause for concern in connection with mechanical fastenings, particularly corrosion caused by the use of dissimilar metals which are widely used in the metal building construction industry. The most severe situation occurs when a conducting electrolyte, such as, for example, acidified water, accumulates at a junction defined between the fastening and the metal framework or profiled sheet. The resulting corrosion is caused by means of a galvanic reaction which occurs between the dissimilar metals.
At present a cadmium coating is often applied to the underlying fastener and this coating acts as both an anti-corrosion coating and as a solid lubricant. However, this is not preferred from a pollution standpoint and therefore a zinc anti-corrosion coating is preferred. This type of coating, however, tends to ball up and jam the drilling and self-tapping effect of the fastener.
In order to overcome the problems of corrosion we have coated a proportion of the screw threads, and sometimes the drill point, of such a fastener with a mastic coating which acts as a sealant at the junction of the screw and the metal framework or profiled sheet so as to prevent the ingress of moist air and which also serves as a protective coating against galvanic corrosion of the fastener. The mastic coating is applied to the fasteners either by means of dipping them point first into a supply of the mastic or by rolling a band of mastic around the screw thread.
Having applied the mastic, the screws are then rapidly passed into an infra-red conveyor oven so as to cure an outer layer thereof and form a skin, and are subsequently cooled by means of an air blast or are water quenched.
If the fasteners are coated and packed without performance of this preliminary curing step they stick together and form an unusable mass. This process is very sensitive to temperature and the curing time. In addition since the resin is partially cured at the skin or surface, curing or cross-linking of the remainder of the mastic continues very slowly and this means that the product has only a limited shelf-life.